Enhanced protein phosphorylation of endogenous protein substrates has been observed in both SV40-transformed human fibroblasts and cell lines derived from human tumors (osteogenic sarcomas), when comparisons were made to normal human fibroblasts. This phenomenon was not shown to be a result of simple differences in growth rate or cell density, nor was it a cAMP-dependent process. Furthermore, inhibition of this enhanced protein phosphorylation in tumor cells could be obvserved by the addition of Zn ions to homogenized tumor cell preparations at concentrations which caused no detectable diminution of phosphorylation in similar preparations of normal human fibroblasts. In addition, when ZnSO4 was added directly to cultures of normal fibroblast or human tumor lines, nearly complete growth inhibition occurred in the tumor lines at zinc concentrations which caused little or no decrease in the growth rate of the normal fibroblasts. It was also noted in preliminary studies that a protein kinase activity was found to be released into the medium of cultured human tumor cell lines, but no such activity was found in the medium of normal human cells. The studies proposed here include (1) exploration of the differential inhibition of normal and tumor cell growth in the presence of Zn ions; (2) studies of the protein kinase activity secreted by tumor cells into their medium for use as a potential tumor cell marker; and (3) biochemical characterization of the protein kinase activity responsible for the enhanced phosphorylation observed in tumor cells.